This invention relates to load pull testing of microwave power transistors using automatic microwave tuners for synthesizing reflection factors (or impedances) and match the transistors (device under test, or DUT) at the input and output at the fundamental and harmonic frequencies.
Modern design of high power microwave amplifiers, oscillators and other active components used in various communication systems requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient and inaccurate for the transistors operating at high power in their highly nonlinear regions and close to saturation, to be described using analytical or numerical models only. Instead the devices must be characterized using specialized test setups under the actual operating conditions.
A popular method for testing and characterizing transistors for high power operation is “load pull” and “source pull”. Load pull or source pull are measurement techniques employing microwave tuners 2, 4 and other microwave test equipment, like a signal source 1, an RF (Radio Frequency) load 5, control computer 6 and digital connections 7, 8, 9 between the computer and the tuners and test equipment. The microwave tuners 2, 4 in particular are used in order to manipulate the RF impedance conditions under which the Device under Test 3 (transistor or DUT), is tested (FIG. 1).
Electro-mechanical tuners [3] are used in most cases for high power load pull testing, because they have several advantages, such as long-term stability, higher handling of RF power, easier operation and lower cost, compared to other type of tuners such as electronic and active tuners.
FIG. 2 shows a front view and cross section of an automatic electro-mechanical tuner using the “slide-screw” tuning concept; a slotted airline (slabline) 15, with two RF connectors 25, 26 at both ends is embedded in a solid housing 16, which also comprises a mobile carriage 18 and means for horizontal drive, typically a lead screw 17; said carriage slides smoothly on polished and grounded shafts 19. The carriage 18 comprises a stepper motor 20, which is powered by a control computer 20a running appropriate software and controls the movement of a precise vertical axis 21. At the bottom end of the axis 22 an appropriate clamp 22a holds the holding pin of the RF probe 23 and secures its exact and repeatable positioning very close to the central conductor 24 of the slabline 15. Moving the probe 23 closer to the central conductor 24 increases the amplitude of the reflection factor, and moving it along the axis of the slabline 15 controls its phase.
A cross section of the ‘slide screw’ tuning mechanism is shown in FIG. 3; in this configuration adjustable metallic obstacles (probes or “slugs”) 10 are inserted into the transmission media of the tuners, which is a slotted coaxial or parallel plate airline (slabline) 11; the capacitive coupling between the vertical probe 10 and the central conductor 12 of the slotted airline (slabline) creates a wideband RF reflection factor (┌), of which the amplitude can be adjusted by inserting the probe 10 further into the slabline and modifying the gap between the probe 10 and the central conductor 12 and therefore changing the value of the capacitance between the central conductor 12 and the probe 10.
The probe 10 is held and guided by the vertical axis 13 of the tuner and is moved vertically 14 by the axis 13, which is driven by a vertical lead screw and computer controlled stepper motors, known in prior art [4, FIG. 3] (FIGS. 2, 3).
High power RF transistors (DUT), for which, due to lack of adequate nonlinear numeric models, load pull testing is a very important characterization method, have very low internal RF impedance Rmin of the order of 1 to 2Ω and sometimes below; the tuners used for load pull testing need to physically match the internal impedance of the transistors, meaning that they must be able to generate such low impedances in a measurement system with typically 50Ω characteristic impedance. This means in many cases the tuners must be able to generate RF reflection factors (┌) between 0.92 and 0.98 or a Voltage Standing Wave Ratio (VSWR) between 24:1 and 99:1;
The following simple relations are used: VSWR=(1+┌)/(1−┌), Rmin=50Ω/VSWR. A reflection factor of ┌=1.0 corresponds to a short circuit or Rmin=0, and no power at all is transferred from the DUT to the load. Ordinary single probe tuners can generate typical VSWR values of the order of 20:1 or less (┌=0.905 or smaller).